feat(spanner): make built-in metrics enabled by default

[sinhasubham]

Make built-in metrics enabled by default

This change inverts the logic for enabling built-in OpenTelemetry metrics. Previously, metrics were disabled by default and could be enabled by setting ENABLE_SPANNER_METRICS_ENV_VAR=true.

With this update, metrics are now enabled by default to provide better out-of-the-box observability for users.

To disable metrics, users must now set the new environment variable:
SPANNER_DISABLE_BUILTIN_METRICS=true

The old ENABLE_SPANNER_METRICS_ENV_VAR is no longer used. Unit tests have been updated to reflect this new opt-out behavior.

BREAKING CHANGE: Built-in metrics are now enabled by default. Users who previously did not set any environment variables will have metrics collection and export turned on automatically after upgrading. To restore the previous behavior and disable metrics, thry have to set the SPANNER_DISABLE_BUILTIN_METRICS environment variable to true.

[gemini-code-assist]

Summary of Changes

Hello @sinhasubham, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the out-of-the-box observability for Spanner users by enabling built-in OpenTelemetry metrics by default. The change shifts the control mechanism from an opt-in model to an opt-out model, simplifying metric collection for most users while still providing a clear way to disable them if needed.

Highlights

• Default Metrics Behavior: Built-in OpenTelemetry metrics are now enabled by default, inverting the previous opt-in behavior to provide better out-of-the-box observability.
• New Opt-Out Mechanism: Users can now disable metrics by setting the new environment variable SPANNER_DISABLE_BUILTIN_METRICS=true.
• Deprecated Environment Variable: The old ENABLE_SPANNER_METRICS_ENV_VAR is no longer used and has been removed from the codebase.
• Breaking Change: This change is a breaking change as metrics will automatically be collected and exported for users who previously did not set any environment variables. To restore the previous behavior, the new SPANNER_DISABLE_BUILTIN_METRICS environment variable must be set to true.

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[gemini-code-assist]

Code Review

This pull request correctly inverts the logic for enabling built-in metrics, making them on by default, which is a great improvement for out-of-the-box observability. The changes to the environment variable and the corresponding updates in the client logic and unit tests are clear and well-executed. I've noted one minor point about a leftover constant that could be cleaned up to improve maintainability.

[surbhigarg92]

Since we are making Metrics default enabled with this changes, please make sure that metrics are not enabled when emulator is enabled and when customers use NoCredentials

Check https://github.com/googleapis/java-spanner/blob/main/google-cloud-spanner/src/main/java/com/google/cloud/spanner/SpannerOptions.java#L2088

[sinhasubham]

yes these cases are getting handled.
Emulator condition: https://github.com/googleapis/python-spanner/blob/main/google/cloud/spanner_v1/client.py#L257

NoCredential user condition: https://github.com/googleapis/python-spanner/blob/main/google/cloud/spanner_v1/client.py#L224-L225
This sets emulator setting for NoCredentials users making them act like emulator which means disabled metrics.

[surbhigarg92]

Please see if we can also add Integration test for these metrics like https://github.com/googleapis/java-spanner/blob/main/google-cloud-spanner/src/test/java/com/google/cloud/spanner/it/ITBuiltInMetricsTest.java#L89

[sinhasubham]

Got a confirmation from @surbhigarg92 that unexpected enabling of metrics for customers who did not set ENABLE_SPANNER_METRICS_ENV_VAR previously is not a breaking change and can be merged. Merging this PR.